1. Field of the Invention
The present invention relates to an exposure apparatus which projects the pattern of an original onto a substrate by a projection optical system, thereby exposing the substrate, and a device manufacturing method.
2. Description of the Related Art
In a lithography process for manufacturing devices such as a semiconductor device and liquid crystal panel, the pattern of a reticle (original) is transferred onto a wafer (substrate), which is coated with a photosensitive agent (photoresist), via a projection optical system.
Amid an increase in the packing density of devices, the substrate micropatterning technique is greatly advancing. Exposure apparatuses called a stepper and scanner play central roles in advancing the micropatterning technique. The stepper is a reduction exposure apparatus which forms an image of the pattern of a reticle on a wafer by a projection optical system, and exposes the wafer by the step & repeat scheme. The scanner is an exposure apparatus which exposes a wafer while synchronously scanning it and a reticle.
TTL measurement is known as a method of relative alignment between a reticle (original) and a wafer (substrate) in the exposure apparatus (Japanese Patent Laid-Open No. 2007-250947). In the TTL measurement, measurement for relative alignment between original-side measurement marks formed on the reticle or a fiducial plate arranged in its vicinity, and substrate-side measurement marks formed on the wafer or its vicinity is performed via the projection optical system. Measurement of this scheme is called TTL measurement. In the TTL measurement, light including the same wavelength as that of the exposure light is generally used as the measurement light.
The resolution limit of the exposure apparatus is proportional to the exposure wavelength and is inversely proportional to the numerical aperture of the projection optical system. In view of this, the exposure apparatus has been developed by shortening the exposure wavelength and increasing the numerical aperture of the projection optical system, in order to improve the resolution limit. Note, however, that the depth of focus of the projection optical system is proportional to the exposure wavelength and is inversely proportional to the square of the numerical aperture of the projection optical system. According to this principle, as the resolution of the exposure apparatus increases, the depth of focus decreases rapidly, so a demand for the accuracy of focusing based on TTL measurement is becoming stricter.
In addition, as the resolution limit improves, the line width of devices decreases, so a demand for the accuracy of alignment on a plane perpendicular to the optical axis, which is based on TTL measurement, is also becoming stricter.
To meet these demands, an image obtained by projecting an alignment pattern used in TTL measurement via the projection optical system is desirably close to a device image.
The exposure apparatus is also being required to attain a high throughput, in order to mass-produce devices in a short period of time. In recent years, an exposure apparatus having two wafer stages, an exposure station for exposing a wafer, and a measurement station for measuring the wafer to obtain information used to align the original and each shot region on the wafer is becoming prevalent. Such an exposure apparatus is called, for example, a twin-stage exposure apparatus. The twin-stage exposure apparatus can measure one wafer while exposing another, thus attaining a high throughput.
In the twin-stage exposure apparatus, the first wafer is measured in the measurement station, and then the wafer stage holding the first wafer moves from the measurement station to the exposure station so that the wafer is exposed while being aligned based on the measurement. While the first wafer is exposed, the second wafer is loaded onto the other wafer stage and measured in the measurement station. After the exposure of the first wafer and the measurement of the second wafer are completed, the wafer stage holding the first wafer moves from the exposure station to the measurement station. At the same time, the wafer stage holding the second wafer moves from the measurement station to the exposure station. The operation of swapping these two wafer stages between the exposure station and the measurement station can be called a swap operation.
An optical measuring device is generally used to control the position of the wafer stage upon driving it. The optical measuring device must continuously measure the measurement surface of the wafer stage. Note, however, that different measuring devices are used for the measurement station and the exposure station in the twin-stage exposure apparatus. To maintain the continuity of the measurement results, the position information of the wafer stage, which is measured in one station (e.g., the measurement station), is passed to a system for controlling the position of the wafer stage in the other station (e.g., the exposure station) when the wafer stage moves to the other station.
Unfortunately, the wafer stage can be hardly moved from one station to the other station without any position error, so a so-called swap error occurs during the swap operation. For this reason, TTL measurement is required to be performed in the exposure station with high measurement accuracy even when a swap error exists. However, along with the advance of the micropatterning of devices, the required measurement accuracy becomes stricter, so a measurement pattern for TTL measurement is miniaturized along with the advance of the micropatterning of devices. Because the miniaturization of the measurement pattern inevitably reduces the measurement possible range, a swap error, if any, makes measurement erroneous, if not impossible.
As a measure for solving this problem, a measuring system different from that for TTL measurement can be provided additionally. However, adding a new measuring system increases the complexity, size, and cost of the apparatus. In addition, a control mechanism, adjustment, calibration, and the like become necessary to correct the difference between the measurement result obtained by the TTL measurement and that obtained by the different measuring system. This increases the time and cost consumed in the manufacture and maintenance of the exposure apparatus.
A situation under which the continuity of the measurement results obtained by the optical measuring devices breaks is not particularly limited to the above-described example, and it is also encountered when, for example, the exposure apparatus or measuring devices are initialized, maintained, or started up. Under these situations, a failure in TTL measurement occurs for the above-described reasons again.